Dysfunction of glutamatergic pathways has been implicated in a number of disease states in the human central nervous system (CNS) including but not limited to schizophrenia, cognitive deficits, dementia, Parkinson disease, Alzheimer disease and bipolar disorder. A large number of studies in animal models lend support to the NMDA hypofunction hypothesis of schizophrenia.
NMDA receptor function can be modulated by altering the availability of the co-agonist glycine. This approach has the critical advantage of maintaining activity-dependent activation of the NMDA receptor because an increase in the synaptic concentration of glycine will not produce an activation of NMDA receptors in the absence of glutamate. Since synaptic glutamate levels are tightly maintained by high affinity transport mechanisms, an increased activation of the glycine site will only enhance the NMDA component of activated synapses.
Two specific glycine transporters, GlyT1 and GlyT2 have been identified and shown to belong to the Na/Cl-dependent family of neurotransmitter transporters which includes taurine, gamma-aminobutyric acid (GABA), proline, monoamines and orphan transporters. GlyT1 and GlyT2 have been isolated from different species and shown to have only 50% identity at the amino acid level. They also have a different pattern of expression in mammalian central nervous system, with GlyT2 being expressed in spinal cord, brainstem and cerebellum and GlyT1 present in these regions as well as forebrain areas such as cortex, hippocampus, septum and thalamus. At the cellular level, GlyT2 has been reported to be expressed by glycinergic nerve endings in rat spinal cord whereas GlyT1 appears to be preferentially expressed by glial cells. These expression studies have led to the suggestion that GlyT2 is predominantly responsible for glycine uptake at glycinergic synapses whereas GlyT1 is involved in monitoring glycine concentration in the vicinity of NMDA receptor expressing synapses. Recent functional studies in rat have shown that blockade of GlyT1 with the potent inhibitor (N-[3-(4′-fluorophenyl)-3-(4′-phenylphenoxy)propyl])-sarcosine (NFPS) potentiates NMDA receptor activity and NMDA receptor-dependent long-term potentiation in rat.
Molecular cloning has further revealed the existence of three variants of GlyT1, termed GlyT-1a, GlyT-1b and GlyT-1c, each of which displays a unique distribution in the brain and peripheral tissues. The variants arise by differential splicing and exon usage, and differ in their N-terminal regions.
The physiological effects of GlyT1 in forebrain regions together with clinical reports showing the beneficial effects of GlyT1 inhibitor sarcosine in improving symptoms in schizophrenia patients suggest that selective GlyT1 inhibitors represent a new class of antipsychotic drugs.
Glycine transporter inhibitors are already known in the art, for example:
(see also Hashimoto K., Recent Patents on CNS Drug Discovery, 2006, 1, 43-53; Harsing L. G. et al., Current Medicinal Chemistry, 2006, 13, 1017-1044; Javitt D. C., Molecular Psychiatry (2004) 9, 984-997; Lindsley, C. W. et al., Current Topics in Medicinal Chemistry, 2006, 6, 771-785; Lindsley C. W. et al., Current Topics in Medicinal Chemistry, 2006, 6, 1883-1896).
Further glycine transporter inhibitors are known from the following documents.
WO 2009024611 describes 4-benzylaminoquinolines of formula:

WO 2009121872 describes tetrahydroisoquinolines of formula:

WO 2010092180 describes aminotetraline derivatives of formula:

WO 2010092181 describes heterocyclic compounds of formula:

WO 2012020131 describes aminoindane derivatives of formula:

WO 2012020130 describes phenalkylamine derivatives of formula:

WO 2012020133 describes tetraline and indane derivatives of formula:

WO 2012152915 describes benzazepine derivatives of formula:

WO 2012020134 describes phenalkylamine derivatives of formulae:

WO 2013020930 describes aminochromane, aminothiochromane and amino-1,2,3,4-tetrahydroquinoline derivatives of formula:

WO 2013072520 describes N-substituted aminobenzocycloheptene, aminotetraline, aminoindane and phenalkylamine derivatives of formulae:

WO 2013120835 describes isoindoline derivatives of formula

It was one object of the present invention to provide further glycine transporter inhibitors. It was a further object of the present invention to provide glycine transporter inhibitors which combine high stability with high affinity. It was a further object of the present invention to provide glycine transporter inhibitors which show favorable efflux properties. It was a further object of the present invention to provide glycine transporter inhibitors which combine high stability and high affinity with favorable efflux properties. It was a further object of the present invention to provide glycine transporter inhibitors which show good oral bioavailability.